Our previous studies had shown that ascorbic acid deficiency in guinea pigs resulted in decreased levels of IGF-I. They also suggested that there was formation of a circulating inhibitor of DNA synthesis in 3T3 cells and of extracellular matrix synthesis in cultured chondrocytes. Experiments in which serum from scorbutic or fasted guinea pigs was mixed with serum from normal guinea pigs confirmed that an inhibitor is present in the experimental sera. In both cell systems, inhibition could be reversed by the addition of IGF-I to the cultures containing scorbutic or fasted sera. The results support our model in which effects of vitamin C deficiency on growth occur because of the fasting state caused by the deficiency. Regulation of growth under these conditions probably results from decreased levels of circulating IGF-I as well as the appearance of an inhibitor of IGF- I activity. We have continued studies to elucidate the mechanisms responsible for the unusual collagen phenotype of 4-nitroquinoline-1-oxide transformed Syrian hamster embryo fibroblasts. These cells fail to synthesize the pro alpha 1 (I) subunit of type I procollagen but continue to synthesize altered forms of the other subunit, pro alpha 2 (I). Present experiments using cell-free translation and hybridization of RNA from normal and transformed fibroblasts with labeled pro alpha 1 (I) DNA probes showed that mRNA of pro alpha 1 (I) is absent from the transformant. Southern blotting of cellular DNA indicated that the gross structure of the pro alpha 1 (I) gene was unchanged by transformation. The polypeptide translated by RNA from the transformant was similar to pro alpha 2 (I). These studies provide a molecular basis for the transformed collagen phenotype.